1. Field of the Invention
The present invention relates to a steering column device incorporating a steering shaft capable of a tilt operation and a telescopic operation of a steering wheel.
2. Description of the Related Art
Some of steering wheels of automobiles are each capable of a tilt operation and a telescopic operation to respectively adjust the vertical position and the anteroposterior position of the steering wheel according to a driver's physical constitution. The vertical position and the anteroposterior position of the steering wheel thus adjusted are fixed by a supporting device with which a steering column for housing a steering shaft is supported onto a vehicle body side.
This supporting device includes a vehicle-body-side bracket having a fixed longitudinal wall portion disposed at a right or left side of the steering column, and a movable longitudinal wall portion provided on an outer tube to be movable relative to the fixed longitudinal wall portion in directions of a tilt operation and a telescopic operation. When an operation lever is operated, a clamping force generated by a clamp shaft brings the fixed longitudinal wall portion and the movable longitudinal wall into pressure contact with each other, and thereby the steering column is fixed. The fixation of the steering column is discontinued by releasing the pressure contact.
In other words, when the steering column is fixed by using the supporting device, relative movement of the fixed longitudinal wall portion and the movable longitudinal wall portion is prevented by friction resistance caused by the pressure contact of both of the wall portions. Here, it is difficult to ensure flat surfaces of both of the fixed longitudinal wall portion and the movable longitudinal wall portion because members of these two wall portions may be distorted. For this reason, the two wall portions may contact each other at unintended regions. If the two wall portions contact each other in the unintended regions, contact between the fixed longitudinal wall portion and the movable longitudinal wall portion is unstable. Such unstable contact has a risk of obtaining insufficient support stiffness. Moreover, resonance frequencies may vary among the vehicle body, the steering wheel and other components located therebetween. In this case, without obtaining a desired resonance frequency, the steering wheel may vibrate by resonating with the vehicle body during idling.
In this regard, Japanese Unexamined Patent Application Publication No. 2007-223383 proposes a supporting device in which upper and lower protrusions each extending in an axial direction of a steering column are formed on an outer side surface of a movable longitudinal wall portion to contact with a fixed longitudinal wall portion, and in which the movable longitudinal wall portion and the fixed longitudinal wall portion are brought into stable pressure contact with each other by use of the upper and lower protrusions.